The invention relates to a method of improving the shelflife of tonometered fluids containing at least one dissolved gas component, and a container of flexible, gas-tight material which unfolds when it is being filled and collapses when it is being drained, preferably a multilayered aluminum laminate bag for receiving and storing tonometered fluid.
For the purpose of calibrating gas sensors it is possible to use the respective gases or tonometered fluids containing the respective gases in dissolved form. If other sensors (such as electrolyte sensors, pH sensors, substrate sensors, etc.) are to be calibrated in addition to the gas sensors in one and the same sensor assembly, and if the same conditions are to be established during calibration as for the analysis of liquid samples, the use of tonometered fluids is recommended.
These may be produced ad hoc by tonometry using precision gases from gas cylinders. This well-established practice has the disadvantage of requiring the use of separate gas cylinders which have to be exchanged at regular intervals and are subject to strict safety regulations.
As a suitable alternative pre-tonometered fluids could be used which are not hampered by the above restrictions. Problems may occur with storing the gassed fluids, above all, if the gas component does not enter a chemical bond but is only present in dissolved form.
It has been known for a long time to store such tonometered fluids in glass ampoules or metal cans, without any significant deviations in the originally set component concentrations during the shelflife of the fluid. Problems will occur during fluid withdrawal upon use in the devices to be calibrated, since the rigid containers require an air inlet for removal of the fluid. Due to the entry of air the gas phase above the fluid will change, leading to a change in the chemical composition of the fluid and the gases dissolved in it.
The situation described above may be improved by using flexible containers, such as plastic bags made from film material. In U.S. Pat. No. 4,116,336, for instance, a bag of flexible material is disclosed, which contains a reference solution for a blood gas analyzer. Avoiding a gas phase, the flexible, gas-tight bag contains a liquid exhibiting known values for pH, PCO2 and PO2 for a given temperature. The partial pressures of the gases are below 600 mmHg at 37xc2x0 C. The reference solution according to U.S. Pat. No. 4,116,336 can contain non-soluble organic substances, such as fluorocarbons, in order to increase oxygen solubility and thus the total amount of oxygen. The flexible bag essentially consists of laminated film and is sealed at the seams.
In WO 97/16309 A1 another flexible bag is described, which contains an oxygen reference solution and is also made from laminated film. The inner layer of the film material, which is next to the reference solution, consists of a sealable polymer, the middle layer is an aluminum foil, and the outer layer is made from polyester. The shelflife of the calibrating solution is predicted to be 61 weeks at room temperature (25xc2x0 C.), with a quoted deviation of 2 percent from the initial values. The change in gas values is a logarithmic function of exterior temperature, however. Thus, the shelflife will be reduced to about 1.3 weeks at 50xc2x0 C., which may happen easily during transport (air transport, warehouses, etc.). A similar product, which is known as xe2x80x9cCal Bxe2x80x9d of Mallinckrodt Sensor Systems Inc, Ann Arbor, Mich., has a shelflife of only 7 weeks at 25xc2x0 C.
Another flexible container is known from WO 93/21533 A1, which contains a calibrating medium with dissolved gases. Helium is used to stabilize the concentrations of the dissolved gases.
In EP 0 724 152 A2 a container is described, which is used for calibrating solutions and/or other fluids used in blood gas analysis. The container presented in one variant of the invention, which is made from flexible and gas-tight material, is provided with a self-sealing connecting element cooperating with a piercing element (hollow needle) of the analyzer.
WO 86/05590 A1 shows a device for the determination of blood parameters. Two tonometered reference solutions are used, for which flexible, gas-impermeable bags are provided, which do not include a gas headspace.
From JP 03-205 533 A2, finally, a device with two flexible bags is known. The entire system comprises an inner bag containing a calibrating solution, which is enclosed by a gas-tight outer bag filled with a gas phase. The inner bag has a defined permeability for CO2 and O2. Since the outer bag is filled with a gas phase the ambient pressure (outside of the bag system) provides no key for determination of the interior pressure of the fluid. Thus it is not possible to calculate the tonometric values. Other drawbacks concern production, as an inner bag and an outer bag must be manufactured, separately filled and separately sealed.
Flexible bags that are ready for calibration have yet another shortcoming as they will not be absolutely gas-tight. In addition, film oxidation may occur, which will also contribute to distorted O2 values. When tonometered fluids are stored in the hitherto described flexible bags there will be the risk of micro bubbles which may form in the fluids if the bags are stored at a pressure below filling pressure (see for instance U.S. Pat. No. 4,116,336).
It is an object of this invention to propose a method of increasing the shelflife of tonometered fluids in flexible containers, and to further develop flexible containers for tonometered fluids such that the problem of the formation of micro bubbles can be mastered.
According to the invention this object is achieved by providing that a gas phase containing at least the dissolved gas component be additionally introduced into the flexible container, the volume of the tonometered fluid and the volume of the gas phase together amounting to less than the maximum filling volume of the flexible container.
Whereas known methods with flexible containers avoid a gas phase in order to permit the simple physical-chemical relationship between partial pressure, solubility, and concentration to be utilized for determination of the partial pressure, this idea is abandoned by the invention, and an additional gas phase is introduced into the flexible container, whose gas component is considerably higher than would be possible with the aqueous solution. As the flexible container will not be completely full, the foil from which the container is made will exert only a negligible force on the contents of the bag during each stage of filling or drainage, such that with a triple-layer laminate bag the pressure difference between interior and exterior pressure will amount to less than 1 mbar. As a consequence, the partial pressure of the gas or gases dissolved in the fluid can be determined by means of the exterior pressure (barometric pressure), which essentially corresponds to the interior pressure, and the temperature.
A container according to the invention, which is made from flexible, gas-tight material and contains a tonometered fluid with at least one dissolved gas component, is thus characterized in that the tonometered fluid occupies a first part of the volume, and that the flexible container holds a gas phase occupying a second part of the volume, which contains at least the gas component dissolved in the fluid, and that the first and second volume parts taken together amount to less than the maximum filling volume of the flexible container. The ratio of the volume parts of the tonometered fluid and the gas phase may be between 1:3 and 3:1, and preferably about 1:1.
If the storage concerns only dissolved tonometered gases, the gas volume compared to the liquid volume should be as large as possible to obtain good buffering. If chemically bonded gases, such as CO2, are to be stored in their tonometered state in addition to dissolved gases, such as O2, greater liquid volumes are required as a rule. It is thus necessary to calculate optimum volume ratios depending on the application concerned.
The plastic bags could be filled with accurately metered volumes of the respective gases and fluids, for example. In a simple variant of the invention it would also be possible, however, that the flexible container be inserted into a receptacle restricting the unfolding of the container to a predefined volume, and that a metered amount of the tonometered fluid be filled into the container, and that the gas phase be subsequently introduced, upon which the flexible container unfolds until the predefined volume is reached. Advantageously, the predefined volume amounts to 30-90%, preferably to 60-75% of the maximum filling volume.